First of all, thanks to all who responded. We bought this boat 16 months ago in Ft Myers, FL and ran it up to Pickwick Lake in MS; about 800 miles. We didn't go far in that time at Pickwick but now we are headed to the Florida Keys and I'm trying to get some good fuel consumption rates going different speeds. Per the above posts, I will try and clarify some things.
1. I calculated the usage based on the number of gals each tank took at fill-up. I tried to fill them the same but understand that this can lead to some discrepancy. We traveled 228 miles over 2 days and took on a total of 212 gals; SB 119 gals and port 93 gals. The 1st day we ran fast at 12.5 mph at 2300 rpm and the 2nd day a lot slower at 9.5 mph at about 1600 rpm.
2. The gear ratio per the survey is 1.96 - 1 for both engines
3. To the best of my knowledge, the running gear is fine and the props are pitched the same.
4. According to the temp gages, the SB runs a little hotter, maybe at 190 vs 185 for the port.
5. I do not have a fuel management system panel. Just levers in the fuel lines for on/off.
6. By making sure the engines are loaded the same, I'm guessing this means ensuring each engine is running at the same RPM.
7. This is where it may get interesting. The tachs were pretty erratic at times. One day one of the bridge tachs was off so I referred to the lower helm tachs, then the next day, the opposite, then the next day either one or the other port or SB tach would be off. I tried to match the RPMs using whichever tach wasn't acting up. Then the next hour they would settle down and seem to be reading accurately.
8. I'm sorry to say I do not know which tank or tanks my return fuel lines go to. The genny does run off the SB but not sure about the return line.
9. The percentage difference between the two tanks seems to be about 23%.
10. This probably doesn't have anything to do with this, but we had a delivery Capt from Ft Myers to Pensacola who ran the genny the whole way (about 40 hrs). The fuel usage at this point was fairly equal. From Pensacola to Pickwick we did not run the genny at all and this was when the fuel usage seems to begin to diverge.
You all have given me some things to check out, especially the tachs, engine temp, and fuel line returns. Thanks again and any more input is much appreciated!!